Emil Briggs (me.delete@this.nowherespam.com) on July 24, 2018 6:30 am wrote:
>
> If I recall correctly Micron launched a product that consisted of DDR4 dimms with a super cap and flash.
> When power goes down the DRAM contents are written to the flash but otherwise it acts as a normal DIMM.
>


https://www.micron.com/~/media/documents/products/product-flyer/nvdimm_flyer.pdf

Besides the fact that these NVDIMMs are small, you have to connect a cable from each DIMM to a HDD sized supercapacitor module, so the inside of your computer case will be quite ugly.


Except that you avoid the need for a custom program for saving and restoring the memory, this is a technical solution which is inferior to just using standard DIMMs and an UPS.

Writing the custom program would be a one-time expense, immediately recovered at the current DRAM prices. Maybe such a program already exists, I have not bothered to search.

Even if none can be found, it cannot require more than a few days of work to write it and to integrate it with a bootloader. This could be done in a way transparent for the OS, e.g. you could write a kernel module, which could be entered e.g. with a ioctl after the UPS signaled the power failure. The kernel module could save a memory & registers image on a SSD, then shut down the computer. The memory and the registers could be restored from the image after power-on by a custom boot loader, which will jump then back to the kernel module (some way to store the jump address at a fixed address would be required), which will return to userland. Thus, except for a jump in time, the OS and all the other programs would not feel that the computer was shut down and the memory content would be as persistent as with NVDIMMs, but at a much lower cost.


The advantage of 3DXP is not that they are persistent (as persistence can be obtained very cheaply), but that they are supposed to be cheap enough so that you should be able to have so much memory that its persistence might be useful.

Unfortunately it does not seem that they would be cheap enough any time soon.

If I could afford to have at least 6 TB (preferably 8 TB) of 3DXP in a computer, then I would retire my NVME SSDs and I would keep only my magnetic tapes above the 3DXP.

Now, when I need speed I use file systems in DRAM. For terabytes of data I used NVME SSDs. I do not have such a desire for simultaneous large size and fast access that would determine me to buy an extra memory layer. On the other hand if I could completely replace a memory layer, I would do that.



I do not doubt that there are applications for which 3DXP might be useful even at the current prices, but in the world of small & medium size companies and also in the world of R&D departments, regardless of company size, I have not encountered such applications.


So I will wait for 3DXP DIMMs to become cheaper.